For related art, reference should be made to U.S. Pat. No. 4,239,954 "Backer for Electron Beam Hole Drilling", Howard et al of common assignee herewith which discloses the general method by which backers are used. In particular the patent discloses backers having liquifiable organic and inorganic binders. Reference should also be made to application Ser. No. 239,276 "Backers Having Glass Particulates for Electron Beam Hole Drilling", filed on even date hereof by J. J. Brennan and L. W. Jordan, referred to hereinafter as "the copending application". Disclosed therein are backers containing certain glasses with novel characteristics, most notably, those having softening temperatures above 700.degree. C. and having both a high temperature constituent which melts and a low temperature constituent which volatilizes. The present invention is an improvement on the invention of the copending application, and involves backers especially designed for drilling superalloys which are used at high temperatures, such as in gas turbines.
Superalloy parts for gas turbines are often electron beam drilled to create small cooling holes. These parts are then used at high temperatures in oxidizing and corrosive atmospheres either in their natural state or with a coating. When coated, the procedure involves heating to a high temperature. Certain residues in contact with the superalloy during high temperature exposure can react or corrode the superalloy. This can degrade mechanical and corrosion resisting properties of the part and lead to premature failure.
In fact, using backers containing sodium silicate binder with soda lime glass particulate is found to produce such corrosion. This was attributed to the difficulties in removing all backer residue from the cavities of articles such as gas turbine airfoils having many interstices, but it was not known what aspects of the backer were influential, or what particular backer compositions should be used.